The number of power plants utilizing geothermal brine as a heat source is increasing annually. In most cases, geothermal artesian steam, or steam flashed from hot geothermal brine, is applied to steam turbines that drive electrical generators. In many locations, the geothermal brine contains a significant amount of dissolved scaling material whose solubility is proportional to the temperature of the solution.
A major concern in many plants is the presence of salts and/or silica in geothermal brine, and the possibility of its precipitation in the various components of the power plant as the brine cools during its flow through the plant, or in piping conveying the spent brine to a reinjection well, or even in the reinjection well itself, if this option is used. Such precipitation fouls, and may even block flow through, various heat exchangers requiring expensive cleaning operations that may involve shut-down of the plant.
Precipitation of salts and/or silica is particularly troublesome when steam is produced from brine by flashing because the flashing operation simultaneously cools the brine and increases its concentration. Consequently, such geothermal power plants are usually limited to medium and high temperature geothermal sources (e.g., above about 160.degree. C.) because the temperature of the brine throughout the plant will be high enough to prevent precipitation in various parts of the power plant, of salts and/or silica which is a primary constituent in most geothermal brines.
In order to utilize lower temperature geothermal sources, it is conventional to transfer heat in geothermal brine via a heat exchanger to an organic fluid thereby cooling the brine and vaporizing the organic fluid which is then expanded in a turbogenerator to produce power and heat depleted vapor. An air or water cooled condenser condenses the vapor exhausted from the turbogenerator and returns the resultant condensate to the heat exchanger, whereupon the cycle repeats. The cooled brine is generally piped to a reinjection well and returned to the ground. In such cases also, the brine is always maintained at a temperature above which salts and/or silica precipitation occurs.
In some cases, the salts and/or silica present in the geothermal brine in the source well is at saturation thus preventing any use of the brine in a power plant because cooling of the brine as it is flashed or as it enters a heat exchanger would result in the precipitation of salts and/or silica. In other cases, the amount of salts and/or silica in the source brine is so high that saturation temperature may be reached in a flash chamber when the brine is flashed, or somewhere in the heat exchanger during the heat exchange process with the organic fluid. In these cases, also, severe maintenance problems are presented.
As a consequence, many geothermal locations having hot brine in excess of 175.degree. C., a temperature with which an organic fluid base power plant is economically feasible, cannot be used for power generation because of the salts and silica content of the brine.
Industrial processes, as well as combustion processes, are another source of hot fluids containing foreign material in the form of particulate that interfere with heat transfer. Examples of such fluids are hot gases containing particulates. In ferro-alloy plants, for instance, hot air exhausted from the plants entrain large quantities of particulate in the form of silica; and recovery from such air of heat which could be utilized for heating or producing steam, electricity, etc. is practical only if the foreign material, i.e., silica entrained in the air, can be treated in a manner that permits ongoing use of the heat. In the prior art, it has been found that, when heat exchangers are used with hot air containing particulate, the latter deposit on the heat transfer surfaces in contact with the gases causing fouling of the heat exchangers. This requires cleaning operations to be carried out regularly. Furthermore, because of the fouling problem, the heat exchangers must be constructed of expensive alloys which adds to the overall expense of the equipment and its maintenance.
Combustion processes used in power production, for example, are a source of hot fluids containing foreign material in the form of precipitates that interfere with heat transfer. The hot fluids can be produced by scrubbing flue gases produced by burning fuel, particularly high-sulfur content coal or oil often used to generate electricity. As is well known, it is conventional to scrub flue-gases in order to remove noxious gases before the flue gases are released to the atmosphere. Conventionally, flue gases, which often contain particulates as well as noxious gases, are scrubbed to remove sulfur dioxide by contacting the gases with a solution containing oxides or hydroxides of calcium or magnesium such that calcium or magnesium sulfate precipitates into the scrubbing solution. The result of this operation is the production of large quantities of hot liquid containing precipitates. The heat in this liquid is difficult to utilize because of the presence of precipitates.
It is therefore an object of the present invention to provide a new and improved method of and means for extracting heat from hot fluids containing foreign material such as salts and/or silica.